A semiconductor device, such as a solid-state image pickup device has been known, in which transfer registers are formed as a plurality of impurity layers extending in lines in such a manner that the impurity layers become deeper toward a transfer direction of charges. In this configuration, charges can be transferred efficiently, as well as the charges can be prevented from being transferred in a backward direction.
It is known that these impurity layers can be formed using a grating mask in which light transmittance is different depending on positions. More specifically, a resist layer on a semiconductor substrate is exposed using the grating mask in which light transmittance is controlled so that the light transmittance gradually increases toward a transfer direction of charges. Subsequently, the exposed resist layers are developed. Then, residual resist films that have portions in which the film thickness gradually decreases toward the transfer direction of charges are formed. When ions are implanted into the semiconductor substrate using the residual resist films as a mask, impurity layers are formed in which the layers gradually become deeper toward the transfer direction of charges in accordance with the film thicknesses of the residual resist films.
However, this conventional manufacturing method of the solid-state image pickup device has a problem in that the film thicknesses of the residual resist films greatly deviate from a desired film thickness. When the film thickness deviates from a desired film thickness, the depths of the impurity layers formed deviate from desired depths depending on the film thickness. Therefore, it is impossible to stably form impurity layers having a desired inner potential.
Specifically, since a potential difference between inner potentials of adjacent impurity layers (step potential) changes, it impossible to stably obtain an effect of accumulating charges and preventing backward flow of the accumulated charges. Accordingly, the efficiency of charge transfer is deteriorated, and a solid-state image pickup device satisfying product characteristics cannot be formed.
Embodiments of the present invention are described in view of resolving the above problem, and it is an object to provide a method for manufacturing a semiconductor device capable of satisfying desired product characteristics even if the film thicknesses of residual resist films vary.